From CH 469 810, a thin-walled steel product in sheet or strip form and a method for its production is known, which can be used for the production of tinplate with a higher strength. The steel product is produced from an unalloyed steel with a carbon content of 0.03-0.25 wt % and has a manganese content of 0.2-0.6 wt % and a silicon content of less than 0.011 wt %. The steel product is characterized by a fine structure consisting at least partially of martensite and ferrite and has tensile strengths of at least 6328 kg/cm2 and an elongation at break of at least 1.5%. For the formation of these characteristics, the steel product is first heated in a furnace to a temperature above the A1 point and subsequently is quenched in a water bath. The tinning of this steel product is then carried out in a known manner after the heating and the quenching of the steel strip in an electrolytic tinning path.
Increasingly, there are higher requirements for the characteristics of metal materials for the production of packagings, in particular with regard to their formability and their strength and their corrosion resistance. So-called dual-phase steels are known from automobile construction, which have a multiphase structure, which essentially consists of martensite and ferrite or bainite, and which, on the one hand, have a high tensile strength and, on the other hand, also have a high elongation at break. Such a dual-phase steel with a yield strength of at least 580 mPa and an elongation at break A80 of at least 10% is, for example, known from WO 2009/021898 A1. As a result of the combination of material characteristics of such dual-phase steels with a high strength and a good formability, these dual-phase steels are particularly suitable for the production of complex-formed and highly stressable components, as they are required, for example, in the area of the chassis for automobiles.
The alloy of the known dual-phase steels is, as a rule, composed of a martensite fraction of 20% to 70%, and any residual austenite fraction and ferrite and/or bainite. The good formability of dual-phase steels is guaranteed by a relatively soft ferrite phase and the high strength is produced by the solid martensite and bainite phases, bound in a ferrite matrix. The desired characteristics with regard to formability and strength can be controlled within broad ranges by the alloy composition of dual-phase steels. Thus, for example, by adding silicon, the strength can be increased by hardening the ferrite or the bainite. By adding manganese, the martensite formation can be positively influenced and the formation of perlite can be prevented. Also, the additives of aluminum, titanium, and boron can increase the strength. The additive of aluminum is, moreover, used for the deoxidation and the binding of nitrogen that may be contained in the steel. For the formation of the multiphase alloy structure, dual-phase steels are subjected to a recrystallizing (or austenitizing) heat treatment, in which the steel strip is heated to such temperatures and subsequently cooled that the desired multiphase alloy structure is adjusted with an essentially ferrite-martensite structure formation. Usually, cold-rolled steel strips are annealed in a recrystallizing manner in a continuous annealing process in the annealing furnace, wherein the parameters of the annealing furnace, such as throughput speed, annealing temperature, and cooling rate, are adjusted in accordance with the required structure and the desired material characteristics.
From DE 10 2006 054 300 A1, a more resistant dual-phase steel and a method for its production are known, wherein a cold- or hot-rolled steel strip is subjected, in the production process, to a recrystallizing continuous annealing in a continuous annealing furnace in a temperature range of 820° C. to 1000° C., and the annealed steel strip is subsequently cooled from this annealing temperature at a cooling rate between 15 and 30° C. per second.
For use as packaging steel, the dual-phase steels known from automobile construction are not suitable as a rule, because they are very expensive, in particular as a result of the high fractions of alloy elements, such as manganese, silicon, chromium, and aluminum, and because for the use of packaging steel in the food sector, for example, some of the known alloy elements may not be used, since a contamination of the food by diffusion of the alloy components into the filler in packages must be ruled out. Furthermore, many of the known dual-phase steels have such a high strength that they cannot be cold-rolled with the systems usually used for the production of packaging steel.
Moreover, packaging steel must have a high corrosion resistance and a good resistance with respect to acids, since the contents of packages made of packaging steel, such as beverage and food cans, are frequently acidic. Packaging steel, therefore, has a metal coating as a protective layer for corrosion, for example, made of tin. The quality of this corrosion protection layer depends, in a very essential manner, on its adhesive capacity on the steel sheet surface. For the improvement of the corrosion resistance of the coating and the adhesion of the corrosion protective layer on the steel sheet surface, for example, in the production of tinplate (tin-plated steel sheet), the tin coating applied on the steel sheet galvanically is melted after the coating process. To this end, the coating deposited galvanically on the steel strip is heated to a temperature lying only slightly above the melting point of the coating material (with a tin coating, for example, to 240° C.) and subsequently is quenched in a water bath. By the melting of the tin coating, the surface of the coating receives a shiny appearance and the porosity of the iron-tin alloy layer between the coating and the steel strip is reduced, wherein its corrosion resistance is increased and its permeability for aggressive substances, for example, organic acids, is reduced.
The melting of the coating can, for example, be carried out by the inductive heating of the coated steel strip or by electrical resistance heating. DE 1 186 158-A, for example, discloses an arrangement for the melting of especially the electrolytically applied coatings on steel strips. From DE 1 177 896, a method for increasing the corrosion protection of metalized iron strips or sheets is known, in which the metal coating, which is especially made of tin, is melted by increasing to a temperature above the melting temperature of the coating material and is exposed, during the crystallization process in the coating material, in the range between the melting temperature and recrystallizing temperature of the coating material, to higher-frequency vibrations. In this way, a crystallization of the coating, which is recognized as disadvantageous, is avoided.
In the known methods for the melting of metal coatings on steel strips or sheets, the entire steel strip or sheet, including the applied coating, is, as a rule, heated to temperatures above the melting temperature of the coating material and subsequently cooled again to normal temperatures, for example, in a water bath. For this, a considerable energy demand is necessary. Since, for the restoration of its original structural state and for the improvement of its formability, the cold-rolled steel strip or sheet must be annealed in a recrystallizing manner before the coating, a heating of the entire steel sheet is carried out twice in the known processes for the production of metal-coated steel sheets—namely, first during the recrystallization annealing of the cold-rolled and still uncoated steel sheet and then after its coating with a metal corrosion protective layer for the melting of the applied coating. This heating of the steel sheet twice thereby takes a large amount of energy and considerably increases the cost of the production process.